PROJECT SUMMARY/ABSTRACT The goals of this project are to validate SIPA1L2 as a genetic modifier of the severity of Charcot-Marie-Tooth type 1A (CMT1A), to understand the normal function of SIPA1L2, and to determine whether modulating SIPA1L2 levels may be a therapeutic strategy for CMT1A. CMT1A is the most common form of inherited peripheral neuropathy, comprising around half of all diagnosed CMT cases, and it is caused by a genetic duplication resulting in the overexpression of peripheral myelin protein 22 (PMP22), leading to a demyelinating peripheral neuropathy. Despite the genetic reproducibility of CMT1A, the clinical severity is variable, and a recent case-only GWAS identified an association between multiple SNPs in SIPA1L2 and the severity of foot dorsiflexion in CMT1A patients. Furthermore, SIPA1L2 was found to be in the same SOX10/EGR2 gene co- expression network as other myelin genes, providing a possible mechanism for its modifier effect and suggesting that the down regulation of SIPA1L2 may be a therapeutic strategy that would result in the down regulation of PMP22, providing a novel treatment for CMT1A. However, given the low overall frequency of CMT1A, additional validation in human cohorts is a challenge, and testing the therapeutic potential of SIPA1L2 as a treatment for CMT1A requires an in vivo system modeling the demyelinating neuropathy. For this, we propose to use the established C3-PMP22 transgenic mouse model of CMT1A. In addition, we have deleted the Sipa1l2 gene from the mouse genome using CRISPR/Cas9 technology. Therefore, we are now able to use these mouse models to better understand the normal function of Sipa1l2 and to test whether changing Sipa1l2 levels will change the severity the demyelinating phenotype of the C3-PMP22 mouse model. We propose two aims. In Aim 1, we will study the loss-of-function phenotype of Sipa1l2 heterozygous and homozygous knockout mice to understand its normal function. We will use a combination of behavioral, neurophysiological and histopathological tests, focusing primarily on the neuromuscular system. We will also perform gene expression analysis by RNAseq to help define pathways that are altered by the loss of Sipa1l2, and to determine if changing Sipa1l2 levels in vivo leads to decreased expression of other myelin genes in the sciatic nerve. In Aim 2, we will combine the Sipa1l2 knockout mice with the C3-PMP22 transgenic model to determine if reducing Sipa1l2 levels changes the severity of the demyelinating phenotype. We will again use behavioral, neurophysiological and histopathological outcomes relevant to CMT1A, as well as gene expression analysis to determine the effects of Sipa1l2 in the C3-PMP22/CMT1A background. We anticipate that reducing Sipa1l2 will result in a decrease in the severity of the phenotype through reduced expression of other myelin genes. Our results to date indicate that the loss of Sipa1l2 on its own does not produce a strong phenotype, making down regulation of SIPA1L2 a more attractive strategy for treating CMT1A. The results of Aim 2 will provide an indication of the potential efficacy of such as approach.